1. Field of the Invention
The present invention relates generally to shot peening and, more particularly, is concerned with a system and method for shot peening reactor vessel penetrations.
2. Background of the Invention
In nuclear power generation, a reactor vessel is the primary vessel wherein heat is created for producing steam. Uranium pellets positioned within the reactor vessel produce a controlled nuclear fission for generating this heat. The cylindrical reactor vessel includes a hemispherical bottom and a flanged, removable upper head. To control the nuclear fission process, a plurality of control rods are selectively inserted into or withdrawn in predetermined fashion from the reactor vessel. Control rods are typically stainless steel tubes encapsulating an absorber material, and extend into the reactor vessel when fully inserted. To accomplish this control rod insertion, a plurality of penetrations (stainless steel tubes) pass through the reactor head and into the interior of the reactor vessel. A tubular shaped thermal sleeve is disposed concentrically inside each penetration and external to the control rods (i.e., between the penetration and the control rod). The thermal sleeve includes a flange at each end. One flange rests on a chamfered end of the penetration tube for support, and the other flange extends beyond the corresponding opposite end of the penetration. This arrangement, in effect, prevents the thermal sleeve from being removed. The thermal sleeve protectively covers the control rods and functions to protect the control rods from acute temperature changes.
The penetrations are welded by conventional methods to the reactor vessel head for structural support. However, this welding, although structurally sound, causes stress on the penetrations adjacent the weldments. It becomes, therefore, desirable to relieve this stress on the penetrations. It is well known in the art that shot peening relieves this type of stress on such tubes.
Although shot peening is efficient, it is not without drawbacks in certain situations. Conventional shot peening methods have drawbacks where a liner (such as the thermal sleeve) surrounds the tube to be shot peened. This is because, typically, there is only an eighth of an inch between the sleeve and the penetration. It is, therefore, difficult to effectively shot peen behind this liner without removing the thermal sleeve and, consequently, causing damage to the thermal sleeve during such removal. Replacing a damaged penetration is costly and time consuming.
Therefore, a need exists for improvements in the construction and mode of operating the shot peening mechanism where a liner covers and conceals the surface to be shot peened.